1. Field of the Invention
The present invention relates to an optical modulator wherein a light is modulated in a space within a laser resonator to attain the light modulation with good efficiency.
2. Description of the Prior Art
Intense, ultrafast, and broad-spectral-width pulses close to white-light called continuum or supercontinuum are generated by passing picosecond laser pulses through a nonlinear optical medium having a nonlinear refractive index.
For spectral broadening, it has been known to utilize a phase modulation wherein the spectral range of the laser is broadened when the laser pulse passes through a nonlinear optical medium. As the laser pulse passes through the medium, it causes a refractive index change. This in turn induces a phase change which causes a frequency sweep within the pulse envelope. This process is called self phase modulation (SPM). There are other processes known as induced phase modulation (IPM) and cross phase modulation (XPM). In IPM, a phase modulation for the laser pulses are performed by the aid of an intense laser pulse serving as pumping light. More specifically, when a weak pulse of a different frequency passes through the medium whose refractive index was changed by an intense laser pulse, the phase of the weak optical field can be modulated by the time variation of the index of refraction originating from the primary intense pulse. In XPM, stimulated Raman scattering, harmonic generation and four photon parametric generation induced by an excited light pulse generate a light whose wavelength is different from the wavelength of the excited light, resulting in generation of a spectral broadened light having a spectrum around the frequency of the excited light. Through the use of such processes, the spectrum of the laser pulse can be broadened.
According to SPM, an amount of phase modulation is dependent on light intensity of the incident light, so that it is necessary to introduce a large peak power laser to effectively broaden the spectrum. According to IMP or XPM, while it is not required that the light to be modulated be of a large peak power, it is necessary that the pumping light be of a large peak power. There is a problem that a high modulation efficiency is not attainable in an optical modulation utilizing a nonlinear effect, because the modulation efficiency is dependent on the peak power of the light.